Focal adhesions (FA) are prominent transmembrane structures of many cells in culture. Individual FA's serve to link the extracellular matrix (ECM) to the actin cytoskeleton, and to initiate a cascade of signalling pathways through integrin receptors. The regulation of these cellular signalling pathways is important with respect to many cellular functions including growth and differentiation, tumorigenesis, wound healing, osseointegration, and the formation of a mineralized matrix. The objective of this study was to evaluate protein tyrosine phosphatase (PTP) activity that may regulate tyrosine phosphorylation in FA's. Using a PTP assay, we have confirmed the observations of Maher (PNAS 90:11177, 1993) that lysates of fibroblasts in suspension have elevated PTP activity when compared with lysates of adherent fibroblasts. Looking for PTP's that may be adhesion regulated, we have identified the cytoplamic PTP, PTPDl, in immunoprecipitates of the FA protein paxillin. We are currently exploring this interaction and the relationship of this PTP to FA's. Microinjection of the cytoplasmic domain of a receptor-type PTP, PTP , (GST fusion protein subclone generously provided by Mathew Thomas, Wash. Univ., St. Louis) into adherent fibroblasts depleted the tyrosine phosphorylation in FA's. However, the FA structure appeared to be stable as judged by staining with anti-vinculin. Our results suggest that the tyrosine phoshorylation in FAs is regulated not only by kinases but also by PTP's.